1. Field of the Invention
The present invention relates to a photoreceptor useful in the field of electrophotography and a method for estimating the durability of the photoreceptor. More particularly, the present invention relates to an electrophotographic organic photoreceptor using an organic photoconductive material and a method for estimating its durability susceptible to the differential hardness of a photosensitive layer that serves as one of building blocks for the photoreceptor.
2. Description of the Prior Art
Heretofore, inorganic photoreceptors have been used as electrophotographic photoreceptors. Each of them uses an inorganic photoconductive material as a photoconductive material, such as selenium, cadmium sulfide, amorphous silicon, or zinc oxide. In recent years, however, some of highly polymerized compounds, such as poly-N-vinylcarbazoles, polyvinyl anthracenes, phthalocyanine compounds, and bisazo compounds, have been studied and developed as organic photo-conductors (OPCs) having their own electric conductivities which vary in response to receiving light. Photoreceptors using at least one of these OPCs (hereinafter, referred as OPC photoreceptors) have been also studied and developed, and thus some of them have been put to practical use. In the following description, any OPC that can be used in the OPC photoreceptor will be also referred as an organic photoconductive material.
The OPC photoreceptor comprises an electrically conductive substrate and an organic photosensitive layer formed thereon. The organic photosensitive layer includes one of the above organic photoconductive materials and consists of two layers, a charge generation layer and a charge transport layer, which have their own functions.
An image formation can be carried out by the electrophotographic process using such an OPC photoreceptor. In general, the image-forming process includes the steps of developing and cleaning. Namely, the process includes the steps of:
uniformly charging a surface of the OPC photoreceptor in the absence of light by causing a corona discharge; PA1 forming a latent image of characters, figures, and the like of a source document on the charged surface of the OPC photoreceptor; PA1 developing the latent image by adhering toner particles to the latent image by virtue of the electric field created by the charges on the photoreceptor; PA1 transferring the developed toner particles on the photoreceptor to paper by corona charging the back of the paper with a charge opposite to that of the toner particles and permanently fixing the image to the paper by melting the toner into the paper surface; and PA1 discharging and cleaning the photoreceptor of any excess toner using coronas, lamps, brushes and/or scraper blades, recovering the photoreceptor for reuse. PA1 the differential hardness corresponds to 20 .mu.m or less change in a width of a scratch formed on the surface every 10 g change in vertical load at a time of perpendicularly loading 10 g or more pressure on the surface when a test for evaluating the differential hardness is performed by moving the photoreceptor to a predetermined position at a constant speed of 30 mm/minute under the load in a direction perpendicular to the surface through a conical indenter to make the scratch on the surface, where the conical indenter is of a sapphire or diamond conical indenter having a conical tip portion with its apex in the shape of a hemispherical with a diameter of 0.01 mm and a conical angle of 90.degree.. PA1 providing a surface of the photoreceptor as a sample, PA1 determining the differential hardness of the sample by moving the photoreceptor to a predetermined position at a constant speed of 30 mm/mines under the load in a direction perpendicular to the surface through a conical indenter to make the scratch on the surface, where the conical indenter is of a sapphire or diamond conical indenter having a conical tip portion with its apex in the shape of a hemispherical with a diameter of 0.01 mm and a conical angle of 90.degree.; and PA1 making the determination that the sample has a good durability if the differential hardness corresponds to 20 .mu.m or less change in a width of a scratch formed on the surface every 10 g change in vertical load at a time of perpendicularly loading 10 g or more pressure on the surface.
Comparing with the inorganic photoreceptor, by the way, the OPC photoreceptor is far superior in membrane-formability, light weight properties, high productivity, and so on, but inferior in sensitivity, durability, and stability to a change in its surroundings. Among these disadvantages, for example, the durability problem relates to the differential hardness of the OPC photoreceptor. The differential hardness of the surface of the OPC photoreceptor varies with physical or chemical conditions including the composition or molecular weight of a resin binder used as a binding agent and the ratio of the resin binder to the photoconductive material. Therefore, it is possible to increase the durability of the OPC photoreceptor by improving the differential hardness thereof by the ways of selecting appropriate materials for preparing each of layers of the OPC photoreceptor and appropriately arranging the configuration of these layers.
Generally, the photosensitive layer is positioned on the top of the OPC photoreceptor, so that the surface of the former is provided as the surface of the latter. Thus, the surface of the photosensitive layer is subject to various stresses from the outside. During the step of development, for example, the OPC photoreceptor is exposed to mechanical stresses such as friction caused by contacting with paper, toner, carrier, or the like. During the cleaning step, furthermore, it is exposed to mechanical stresses such as pressure by the cleaner and friction caused by contacting with fine particles (such as paper debris, residual toner particles, and residual carrier particles) being remained on the surface of the photosensitive layer. If the differential hardness of the OPC photoreceptor is low while the cleaning power is strong, the photosensitive layer is easily subject to the effects of the above fine particles on the surface of the photoreceptor. If the photosensitive layer is made of a breakable firm material, furthermore, the impact of producing friction with the fine particles causes a local brittle fracture (micro-fracture) in the surface of the photosensitive layer. This kind of trouble leads to a defective image formation at the time of the development in the next rotation of the OPC photoreceptor (in the form of a drum). Suppose the step of cleaning is completed even though the toner particles remain in a defective (fractured) portion of the drum's surface, the black dot defect in the image is produced.
Incidentally, the conventional cleaning methods include a blade cleaning method and a magnetic blush cleaning method. Among them, the blade cleaning method may be principally applied for the photosensitive layer having a small differential hardness. For preventing the photosensitive layer from causing the local brittle fracture, a man skilled in the art uses a low hardness cleaning blade or applies lubricant powder (setting powder) on a surface of the photosensitive layer to reduce the friction between the photoreceptor and the blade. However, the low hardness cleaning blade increases the possibility of causing the troubles such as a blurred image running in hot and humid surroundings. Even though the lubricant powder is applied, an image defect can be occurred as a result of causing the local brittle fracture when the efficacy of the lubricant powder is decreased as a result of repeating the printing process many times.